1.2.2 Derived Quantities and Units

- In physics, we come across a large number of quantities like speed, momentum, resistance, conductivity, etc. which depend on some or all of the seven fundamental quantities and can be expressed in terms of these quantities.

- These are called derived quantities and their units, which can be expressed in terms of the fundamental units, are called derived units.

- For example, Unit of momentum = (Unit of mass)×(Unit of velocity) = kg m/s = kg m/s

- The above two units are derived units.

- Supplementary units : Besides, the seven fundamental or basic units, there are two more units called supplementary units:

(i) Plane angle and

(ii) Solid angle

- (i) Plane angle (d) :

This is the ratio of the length of an arc of a circle to the radius of the circle.

It is measured in radian (rad).

- (ii) Solid angle (d) :

This is the 3-dimensional analogue of d and is defined as the area of a portion of surface of a sphere to the square of radius of the sphere.

It is measured in steradians